A person who gets stoned with a large amount of bud but however, he or she is still at their full capacity to do certain tasks without an issue. A functional stoner can go to work or a public place and act completely normal as if they're not stoned.

John: You seen the video where Bob Marley smokes a lot of bud and then get back in the studio?
David: Yea damn how was he able to pull that off?
John: Dude, he is a functional stoner that's why